No evidence for isotope discrimination of tritiated glucose tracers in measurements of glucose turnover rates in man

Plasma IL-6 concentration is inversely related to insulin sensitivity, and acute-phase proteins associate with glucose and lipid metabolism in healthy subjects

AIM

It has been shown that atherosclerosis is an inflammatory disease. Recent data suggest that inflammation precedes type 2 diabetes. Hence, we wanted to study the interrelationship between IL-6, insulin sensitivity, lipids and numerous acute-phase proteins.

METHODS

Twenty-one healthy individuals [16 males/5 females, age 27.9+/-1.8 years, body mass index (BMI) 24.1+/-0.8 kg/m(2)] participated in the study. Each patient went through a 4-h hyperinsulinaemic (40 mU/m(2)/min) euglycaemic clamp and 4-h saline infusion. Blood samples were taken before and at the end of the infusions.

RESULTS

Plasma interleukin (IL)-6 concentration correlated inversely with insulin sensitivity (M-value) (r=-0.49, p<0.05). Moreover, the plasma levels of IL-6 associated with c-peptide (r=0.49, p<0.05), fat% (r=0.43, p<0.05) and diastolic blood pressure (r=0.46, p<0.05). alpha-1-acid glycoprotein was related to HbA1(c) (r=0.47, p<0.05), insulin (r=0.55, p<0.01), diastolic blood pressure (r=0.58, p<0.01), systolic blood pressure (r=0.58, p<0.01) and triglycerides (r=0.58, p<0.01). Haptoglobin was correlated with insulin (r=0.46, p<0.05), total cholesterol (r=0.61, p<0.01), BMI (r=0.58, p<0.01), fat% (r=0.63, p<0.01) and lipid oxidation during clamp (r=0.43, p<0.05). Diastolic blood pressure decreased during the clamp (from 78.3+/-1.9 to 72.1+/-2.0 mmHg, p=0.001). Insulin infusion did not affect the serum levels of most acute-phase proteins.

CONCLUSIONS

Our study suggests that low grade inflammation, as reflected by IL-6, A1GP and haptoglobin contributes to the regulation of insulin sensitivity, lipid metabolism and blood pressure in normal human physiology.

Carbohydrate depletion has profound effects on the muscle amino acid and glucose metabolism during hyperinsulinaemia

AIM

We investigated the effect of carbohydrate availability and euglycaemic hyperinsulinaemia on intramuscular and plasma amino acids in 14 healthy men (age 26.5 +/- 0.9 years, b.m.i. 22.9 +/- 0.5 kg/m2).

METHODS

Insulin was infused (1.5 mU/kg/min) for 240 min both after a carbohydrate depleting exercise and after carbohydrate loading. Muscle samples were taken before and after hyperinsulinaemia. Plasma and intramuscular amino acid concentrations were measured.

RESULTS

Insulin-mediated glucose disposal was similar after carbohydrate depletion (65.2 +/- 1.9 micromol/kg/min) and loading (66.9 +/- 2.8 micromol/kg/min). Carbohydrate depletion was associated with decreased alanine and increased branched chain amino acid (BCAA) concentrations in muscle and plasma. Blood lactate was lower after carbohydrate depletion (477 +/- 25 micromol/l) than loading (850 +/- 76 micromol/l, p < 0.001). In carbohydrate depletion, hyperinsulinaemia resulted in a greater increase in intramuscular (from 927 +/- 48 nmol/g muscle to 2029 +/- 104 nmol/g muscle, p < 0.001), than plasma (from 197 +/- 6.7 micromol/l to 267 +/- 11 micromol/l, p < 0.001) alanine. After carbohydrate loading muscle alanine did not rise significantly (from 1546 +/- 112 nmol/g muscle to 1781 +/- 71 nmol/g muscle) whereas plasma alanine decreased (from 339 +/- 26 micromol/l to 272 +/- 13 micromol/l, p < 0.05).

CONCLUSIONS

(1) Carbohydrate availability has profound effects on the interrelationship between glucose and amino acid metabolism and on the form of storage for glucose-derived carbons. (2) For most amino acids changes in plasma levels of amino acids are not related to changes in concentrations of intramuscular amino acids during hyperinsulinaemia.

Direct measurement of the lumped constant for 2-deoxy-[1-(14)C]glucose in vivo in human skeletal muscle

The lumped constant (LC) is used to convert the clearance rate of 2-deoxy-D-glucose (2-DG(CR)) to that of glucose (Glc(CR)). There are currently no data to validate the widely used assumption of an LC of 1.0 for human skeletal muscle. We determined the LC for 2-deoxy-[1-(14)C]glucose (2-DG) in 18 normal male subjects (age, 29+/- 2 yr; body mass index, 24.8+/-0.8 kg/m(2)) after an overnight fast and during physiological (1 mU x kg(-1) x min(-1) insulin infusion for 180 min) and supraphysiological (5 mU x kg(-1) x min(-1) insulin infusion for 180 min) hyperinsulinemic conditions. Normoglycemia was maintained with the euglycemic clamp technique. The LC was measured directly with the use of a novel triple tracer-based method. [3-(3)H]glucose, 2-[1-(14)C]DG, and [(12)C]mannitol (Man) were injected as a bolus into the brachial artery. The concentrations of [3-(3)H]glucose and 2-[1-(14)C]DG (dpm/ml plasma) and of Man (micromol/l) were determined in 50 blood samples withdrawn from the ipsilateral deep forearm vein over 15 min after the bolus injection. The LC was calculated by a formula involving blood flow calculated from Man and the Glc(CR) and 2-DG(CR). The LC averaged 1.26+/-0.08 (range 1.06-1.43), 1.15+/-0.05 (0.99-1.39), and 1.18+/-0.05 (0.97-1.37) under fasting conditions and during the 1 and 5 mU x kg(-1). min(-1) insulin infusions (not significant between the different insulin concentrations, mean LC = 1.2, P<0.01 vs. 1.0). We conclude that, in normal subjects, the LC for 2-DG in human skeletal muscle is constant over a wide range of insulin concentrations and averages 1. 2.

Effects of insulin on glucose turnover rates in vivo: isotope dilution versus constant specific activity technique

The conventional isotope dilution technique was compared with the more accurate constant specific activity (SA) method at six different insulin levels. Paired euglycemic clamp studies were performed in 30 normal subjects (4-hour insulin infusion: 5, 10, 20, 40, 80, and 160 mU . m-2 . min-1) using primed-constant 3-3H-glucose infusion and either conventional unlabeled glucose infusates (Cold-GINF) or labeled glucose infusates (Hot-GINF) to maintain constant SA. At all insulin levels, both glucose disappearance (Rd) and hepatic glucose production (HGP) were underestimated by the conventional technique, and errors during the first 2 hours correlated with glucose infusion rates (GIRs) (r = .93, P < .00001). During the second hour, mean underestimation of HGP varied from 20% +/- 9% to 84% +/- 16% of basal rates from low-dose to high-dose insulin infusion studies. During prolonged equilibration (3 to 4 hours), errors decreased but were still significant in the two low-dose insulin infusion protocols during the fourth hour. In conclusion, using the conventional isotope dilution technique, suppression of glucose production was overestimated and stimulation of glucose Rd was underestimated, and these errors were greater the higher the GIR. Thus, artifactually greater hepatic and smaller peripheral effects may have been assumed for factors or therapies that influence insulin sensitivity in previous studies using a conventional isotope dilution technique, and therefore, reevaluation of these issues may be relevant in future studies.

Androgens and insulin resistance in type 1 diabetic men

OBJECTIVE

In healthy men, both high and low serum testosterone concentrations are associated with insulin resistance, whereas low concentration of sex hormone binding globulin (SHBG) is related to reduced insulin sensitivity. The aim of our study was to examine the association of sex hormones, SHBG, dehydroepiandrosterone (DHEAS) and insulin-like growth factor binding protein-1 (IGFBP-1) on insulin sensitivity in type 1 diabetic patients.

PATIENTS

We examined 23 male patients with the mean age of 29 +/- 1 years, body mass index 22.9 +/- 0.4 kg/m2, insulin dose 47 +/- 3 units/day, glycosylated haemoglobin (HbA1c) 7.8 +/- 0.3% and duration of diabetes 13 +/- 1 years.

DESIGN

Each patient was studied with a 4-hour euglycaemic (5.5 +/- 0.1 mmol/l), hyperinsulinaemic (612 +/- 26 pmol/l) clamp with indirect calorimetry. Muscle biopsies (quadriceps femoris) for the determination of glycogen synthase were performed in 15 patients before and at the end of the clamp.

RESULTS

Insulin infusion reduced the concentrations of IGFBP-1 by 90% (P < 0.001), DHEAS by 11% (P < 0.001), and SHBG by 4% (P < 0.01), whereas free or bound testosterone levels remained unchanged. The fall in IGFBP-1 level was closely related to the basal concentration (r = 0.99, P < 0.001). Basal SHBG concentration correlated directly with total (r = 0.51, P < 0.05) and non-oxidative glucose disposal (r = 0.41, P < 0.05), and with the decrease in lipid oxidation (r = 0.47, P < 0.05) during insulin infusion. The fall in SHBG was inversely related to the mean (30-240 min) FFA concentration during hyperinsulinaemia (r = -0.64, P < 0.001). The fractional activity of glycogen synthase at the end of insulin infusion correlated directly with fasting SHBG (r = 0.71, P < 0.01) and DHEAS concentrations (r = 0.67, P < 0.01).

CONCLUSIONS

In male type 1 diabetic patients: (1) acute hyperinsulinaemia decreases IGFBP-1, DHEAS and SHBG concentrations with the greatest decline in IGFBP-1, (2) SHBG concentration is positively associated with factors indicating good insulin sensitivity, (3) association between fuel homeostasis and SHBG, DHEAS and insulin antagonists suggests a network of these factors in the regulation of insulin action in type 1 diabetic patients.

Failure of complete suppression of endogenous glucose production by euglycaemic hyperinsulinaemia in normal humans

In normal subjects, endogenous glucose production (EGP) is usually assumed to be completely suppressed during euglycaemic clamp studies performed at high insulin levels (> 100 mU L-1). However, this assumption is based on non-steady-state tracer measurements of EGP which are prone to negative errors. We have used purified [6-(3)H]glucose in an optimal tracer infusion protocol to assess the suppression of EGP during 4 h euglycaemic clamps in eight normal men. An insulin infusion rate of 5 mU kg-1 min-1 was chosen to achieve supraphysiological (> 500 mU L-1) plasma insulin concentrations. Using a labelled exogenous glucose infusion, plasma glucose (mean +/- SEM 5.3 +/- 0.1 mmol L-1) and glucose specific activities (mean 100 +/- 3% of basal) were maintained constant from 80 to 240 min. During hyperinsulinaemia, isotopically determined glucose appearance rates (Ra) were greater than glucose infusion rates (GIR) throughout the euglycaemic clamp period (P < 0.001) and EGP (Ra-GIR) was always greater than zero. In seven of the eight subjects studied EGP was partly suppressed but showed a wide variation (EGP 5 to 91% of basal at 80-120 min and 12 to 87% of basal at 200-240 min) while in one subject EGP rose above basal (by 72% at 80-120 min and 49% at 200-240 min). We conclude that EGP is not completely suppressed during euglycaemic clamps at high insulin levels.

Different alterations in the insulin-stimulated glucose uptake in the athlete's heart and skeletal muscle

Physical training increases skeletal muscle insulin sensitivity. Since training also causes functional and structural changes in the myocardium, we compared glucose uptake rates in the heart and skeletal muscles of trained and untrained individuals. Seven male endurance athletes (VO2max 72 +/- 2 ml/kg/min) and seven sedentary subjects matched for characteristics other than VO2max (43 +/- 2 ml/kg/min) were studied. Whole body glucose uptake was determined with a 2-h euglycemic hyperinsulinemic clamp, and regional glucose uptake in femoral and arm muscles, and myocardium using 18F-fluoro-2-deoxy-D-glucose and positron emission tomography. Glucose uptake in the athletes was increased by 68% in whole body (P < 0.0001), by 99% in the femoral muscles (P < 0.01), and by 62% in arm muscles (P = 0.06), but it was decreased by 33% in the heart muscle (P < 0.05) as compared with the sedentary subjects. The total glucose uptake rate in the heart was similar in the athletes and control subjects. Left ventricular mass in the athletes was 79% greater (P < 0.001) and the meridional wall stress smaller (P < 0.001) as estimated by echocardiography. VO2max correlated directly with left ventricular mass (r = 0.87, P < 0.001) and inversely with left ventricular wall stress (r = -0.86, P < 0.001). Myocardial glucose uptake correlated directly with the rate-pressure product (r = 0.75, P < 0.02) and inversely with left ventricular mass (r = -0.60, P < 0.05) or with the whole body glucose disposal (r = -0.68, P < 0.01). Thus, in athletes, (a) insulin-stimulated glucose uptake is enhanced in the whole body and skeletal muscles, (b) whereas myocardial glucose uptake per muscle mass is reduced possibly due to decreased wall stress and energy requirements or the use of alternative fuels, or both.

Non-esterified fatty acids regulate lipid and glucose oxidation and glycogen synthesis in healthy man

We examined the interrelationship of lipid and glucose metabolism in the basal state and during insulin stimulus in 19 healthy men (27 +/- 2 years, body mass index 23.6 +/- 0.6 kg/m2). In each subject, we performed a 4-h euglycaemic (5.3 +/- 0.1 mmol/l) hyperinsulinaemic (647 +/- 21 pmol/l) insulin clamp with indirect calorimetry in the basal state and during insulin infusion, and muscle biopsies before and at the end of the clamp. In the basal state, serum non-esterified fatty acid levels correlated directly with lipid oxidation (r = 0.56, p < 0.05) and indirectly with glucose oxidation (r = -0.80, p < 0.001). Lipid and glucose oxidation rates were inversely related in the basal state (r = -0.47, p < 0.05) and during insulin infusion (r = -0.65, p < 0.01). Basal lipid oxidation and glycogen synthase total activity correlated inversely (r = -0.54, p < 0.05). Lipid oxidation both in the basal state (r = -0.61, p < 0.01) and during insulin infusion (r = -0.62, p < 0.05) was inversely related to muscle glycogen content after the insulin clamp. Fasting plasma triglyceride concentration correlated directly to fasting insulin (r = 0.55, p < 0.05) and C-peptide (r = 0.50, p < 0.03) concentrations and inversely to non-oxidative glucose disposal rate at the end of clamp (r = -0.54, p < 0.05).

IN CONCLUSION

1) Serum non-esterified fatty acid concentration enhances lipid and reduces glucose oxidation. 2) Lipid oxidation is inversely related to total glycogen synthase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of glucagon to stimulate hepatic glycogenolysis in well-nourished patients with mild cirrhosis

The ability of glucagon to stimulate hepatic glucose production (HGP) was studied in clinically stable cirrhotic patients (n = 8) who had, based on long-term follow-up evaluation, relatively good liver function (Child-Pugh A) and whose dietary intake and physical characteristics were comparable to those of healthy control subjects (n = 8). Plasma glucagon concentration was slightly but not significantly increased in cirrhotic patients versus control subjects in the basal state (190 +/- 41 v 126 +/- 24 pg/mL, P = NS) and during a continuous 180-minute glucagon infusion at 3 ng/kg/min (349 +/- 56 v 243 +/- 37, P = NS). The increment in plasma glucagon level (+164 +/- 57 v +127 +/- 35, P = NS) also was slightly greater in the cirrhotic group. HGP (measured with [6-3H]-glucose) in the basal state was similar in cirrhotic and control subjects (1.79 +/- 0.09 v 1.94 +/- 0.15 mg/kg/min, P = NS). In cirrhotic patients, stimulation of HGP by glucagon was blunted during the first 15 to 30 minutes of the infusion period (representing glucagon's predominant effect on glycogenolysis; 0.23 +/- 0.20 v 1.06 +/- 0.19 mg/kg/min, P < .05), but it was not different from that in control subjects during the remaining course of the experiment (30 to 180 minutes). Basal plasma insulin and C-peptide concentrations did not change from baseline during the glucagon infusion in cirrhotics, whereas they increased slightly but not significantly in controls. These data demonstrate that even in the early stages of cirrhosis, the liver is resistant to the stimulatory effect of glucagon on hepatic glycogenolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Action of insulin on glucose metabolism in vivo

Insulin plays a key role in the maintenance of normal glucose tolerance by suppressing endogenous glucose production during a meal. Insulin is not, however, involved in the regulation of splanchnic glucose uptake. The latter process appears, based on studies performed in dogs, to be regulated primarily by the arterial-portal glucose gradient and to a smaller extent by glucose mass-action. Regarding peripheral glucose utilization, insulin is not needed to maintain a normal rate of glucose utilization since this can also be achieved by hyperglycaemia and glucose mass-action. Insulin is, however, necessary for the maintenance of normal rates of glucose oxidation and storage in insulin-sensitive tissues, and for the prevention of excessive gluconeogenic substrate production.

Mechanism of enhanced insulin sensitivity in athletes. Increased blood flow, muscle glucose transport protein (GLUT-4) concentration, and glycogen synthase activity

We examined the mechanisms of enhanced insulin sensitivity in 9 male healthy athletes (age, 25 +/- 1 yr; maximal aerobic power [VO2max], 57.6 +/- 1.0 ml/kg per min) as compared with 10 sedentary control subjects (age, 28 +/- 2 yr; VO2max, 44.1 +/- 2.3 ml/kg per min). In the athletes, whole body glucose disposal (240-min insulin clamp) was 32% (P < 0.01) and nonoxidative glucose disposal (indirect calorimetry) was 62% higher (P < 0.01) than in the controls. Muscle glycogen content increased by 39% in the athletes (P < 0.05) but did not change in the controls during insulin clamp. VO2max correlated with whole body (r = 0.60, P < 0.01) and nonoxidative glucose disposal (r = 0.64, P < 0.001). In the athletes forearm blood flow was 64% greater (P < 0.05) than in the controls, whereas their muscle capillary density was normal. Basal blood flow was related to VO2max (r = 0.63, P < 0.05) and glucose disposal during insulin infusion (r = 0.65, P < 0.05). The forearm glucose uptake in the athletes was increased by 3.3-fold (P < 0.01) in the basal state and by 73% (P < 0.05) during insulin infusion. Muscle glucose transport protein (GLUT-4) concentration was 93% greater in the athletes than controls (P < 0.01) and it was related to VO2max (r = 0.61, P < 0.01) and to whole body glucose disposal (r = 0.60, P < 0.01). Muscle glycogen synthase activity was 33% greater in the athletes than in the controls (P < 0.05), and the basal glycogen synthase fractional activity was closely related to blood flow (r = 0.88, P < 0.001).

IN CONCLUSION

(a) athletes are characterized by enhanced muscle blood flow and glucose uptake. (b) The cellular mechanisms of glucose uptake are increased GLUT-4 protein content, glycogen synthase activity, and glucose storage as glycogen. (c) A close correlation between glycogen synthase fractional activity and blood flow suggests that they are causally related in promoting glucose disposal.

Contribution of muscle and liver to glucose-fatty acid cycle in humans

To examine the influence of elevated free fatty acid (FFA) levels on hepatic glucose production (HGP) and oxidative and nonoxidative pathways of glucose metabolism, 12 healthy subjects participated in two euglycemic insulin-clamp studies performed with and without infusion of Intralipid plus heparin. To elucidate the role of skeletal muscle in this putative interaction, we performed muscle biopsies for the measurement of activities of glycogen synthase (GS), pyruvate dehydrogenase (PDH), and carnitine palmitoyltransferase (CPT). Infusion of Intralipid plus heparin caused an increase in plasma FFA concentrations and rate of lipid oxidation (measured by indirect calorimetry) that was not inhibited by insulin. Suppression of HGP by insulin was impaired by elevated plasma FFA levels. Furthermore, the increase in plasma FFA was associated with a 20% reduction in total glucose metabolism (P < 0.01), which was completely accounted for by a reduction in the rate of glucose oxidation. Although the fractional activity of GS was increased by insulin, elevation of plasma FFA had no influence on this key enzyme of glycogen synthesis. In addition, the activities of PDH and CPT were uninfluenced by the elevation of FFA, suggesting that oxidative processes in skeletal muscle were not a major target for the operative glucose-fatty acid cycle under the current conditions. Taken together, the data indicate that the interaction between FFA and glucose metabolism also involves impaired suppression of HGP by insulin.

Lowering of triglycerides by gemfibrozil affects neither the glucoregulatory nor antilipolytic effect of insulin in type 2 (non-insulin-dependent) diabetic patients

Hypertriglyceridaemia and insulin resistance are closely associated but it is unknown whether hypertriglyceridaemia per se contributes to insulin resistance. In the present study we examined whether gemfibrozil, by lowering triglyceride levels, improves the glucoregulatory and antilipolytic action of insulin in Type 2 (non-insulin-dependent) diabetes mellitus. Twenty patients were randomly allocated to receive either placebo or gemfibrozil 1200 mg daily for 12 weeks in a double-blind study. Very low density lipoprotein triglyceride levels decreased in the gemfibrozil group by 42 +/- 12% (p < 0.01). Gemfibrozil had no effect on the diurnal concentration of non-esterified fatty acids (NEFA). At the randomization HbA1c levels were comparable (7.6 +/- 0.3 vs 7.8 +/- 0.2%, NS) and increased slightly both in the gemfibrozil (8.2 +/- 0.4%, p < 0.05) and placebo groups (8.0 +/- 0.3%, NS). Pre- and post-treatment diurnal glucose and insulin concentrations remained unchanged. Basal pre- and post-treatment hepatic glucose production rates were comparable in both groups and similarly suppressed by insulin. Rate of whole body glucose disposal during a low-dose insulin infusion (serum insulin -90 pmol/l) (pre- vs post-gemfibrozil 11.9 +/- 1.1 vs 11.1 +/- 0.7, pre- vs post-placebo 9.9 +/- 1.1 vs 10.8 +/- 0.8 mumol.kg-1.min-1, NS for both) and a high-dose insulin infusion (serum insulin approximately 500 pmol/l) (16.2 +/- 1.7 vs 17.7 +/- 2.7, 17.1 +/- 4.2 vs 17.4 +/- 2.9 mumol.kg-1 x min-1, respectively, NS for both) remained unchanged. Basal pre- and post-treatment NEFA turnover rates were comparable in both groups and similarly suppressed by insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of glucose turnover rates in euglycaemic clamp studies using primed-constant [3-3H]-glucose infusion and labelled or unlabelled glucose infusates

Underestimation of glucose turnover rates has been a problem in clamp studies using primed-constant [3-3H]-glucose infusion technique. Due to slow mixing in interstitial compartments concealed specific activity gradients may arise between plasma and interstitial compartments during intravenous unlabelled glucose infusion. Such specific activity gradients, however, can be prevented if plasma specific activity is maintained constant. Two euglycaemic clamp studies (insulin infusion 40 mU m-2 min-1) were performed in six lean normal subjects. Using conventional unlabelled glucose infusates plasma specific activity declined by 74%, tracer determined glucose appearance was smaller than actual glucose infusion rates (317 +/- 11 vs 366 +/- 15 mg m-2 min-1, p < 0.001), and erroneous negative values were calculated for glucose production (- 49 +/- 7 mg m-2 min-1). Average underestimation during the first 2 h correlated with glucose infusion rates (r = 0.88, p < 0.02). In contrast, when plasma specific activity was maintained constant, using appropriately labelled glucose infusates, tracer determined glucose appearance and glucose infusion rates were similar (385 +/- 16 vs 385 +/- 17 mg m-2 min-1), and negative errors for glucose production were avoided. In conclusion, using unlabelled glucose infusates, as in previous studies, suppression of glucose production is overestimated and stimulation of glucose utilization is underestimated. As errors were greater with larger glucose infusions, the mistakes may have been greatest in insulin sensitive control subjects, and smaller in insulin resistant subjects. Therefore, re-evaluation of hepatic insulin sensitivity seems appropriate in diabetes, obesity, and other insulin resistant states.

Incomplete suppression of hepatic glucose production in non-insulin dependent diabetes mellitus measured with [6,6-2H2]glucose enriched glucose infusion during hyperinsulinaemic euglycaemic clamps

We have minimized methodological errors in the isotope dilution technique by using stable isotope, [6,6-2H2]glucose, thus avoiding the problem of contamination of tritiated glucose tracers and, by maintaining a constant plasma tracer enrichment have reduced error due to mixing transients. Using these modifications we have calculated hepatic glucose production in 20 patients with non-insulin-dependent diabetes mellitus during low (1 mU kg-1 min-1) and high (8 mU kg-1 min-1) dose insulin infusions. Mean fasting hepatic glucose production was 14.2 +/- 0.8 mumol kg-1 min-1. This suppressed by only 68% to 4.6 +/- 0.8 mumol kg-1 min-1 during the low-dose insulin infusion (plasma insulin 0.85 +/- 0.05 nmol l-1) and did not suppress further during the high-dose insulin infusion (plasma insulin 14.55 +/- 0.83 nmol l-1). Hepatic glucose production was significantly higher than zero throughout the study. Thus, we have found that minimization of known errors in the isotope dilution technique results in physiologically plausible and significantly positive values for hepatic glucose production indicating that the liver is resistant to insulin in patients with non-insulin-dependent diabetes mellitus.

Effect of physiologic hyperinsulinemia on glucose and lipid metabolism in cirrhosis

Insulin secretion and insulin sensitivity were evaluated in eight clinically stable cirrhotic patients and in 12 controls. OGTT was normal in cirrhotics but plasma insulin response was increased approximately twofold compared with controls. Subjects received a three-step (0.1, 0.5, 1.0 mU/kg.min) euglycemic insulin clamp with indirect calorimetry, [6-3H]-glucose, and [1-14C]-palmitate. During the two highest insulin infusion steps glucose uptake was impaired (3.33 +/- 0.31 vs. 5.06 +/- 0.40 mg/kg.min, P less than 0.01, and 6.09 +/- 0.50 vs. 7.95 +/- 0.52 mg/kg.min, P less than 0.01). Stimulation of glucose oxidation by insulin was normal; in contrast, nonoxidative glucose disposal (i.e., glycogen synthesis) was markedly reduced. Fasting (r = -0.553, P less than 0.01) and glucose-stimulated (r = -0.592, P less than 0.01) plasma insulin concentration correlated inversely with the severity of insulin resistance. Basal hepatic glucose production was normal in cirrhotics and suppressed normally with insulin. In postabsorptive state, plasma FFA conc (933 +/- 42 vs. 711 +/- 44 mumol/liter, P less than 0.01) and FFA turnover (9.08 +/- 1.20 vs. 6.03 +/- 0.53 mumol/kg.min, P less than 0.01) were elevated in cirrhotics despite basal hyperinsulinemia; basal FFA oxidation was similar in cirrhotic and control subjects. With low-dose insulin infusion, plasma FFA oxidation and turnover failed to suppress normally in cirrhotics. During the two higher insulin infusion steps, all parameters of FFA metabolism suppressed normally. In summary, stable cirrhotic patients with normal glucose tolerance exhibit marked insulin resistance secondary to the impaired nonoxidative glucose disposal. Our results suggest that chronic hyperinsulinism may be responsible for the insulin resistance observed in cirrhosis.

Coronary heart disease risk factor profiles in black patients with non-insulin-dependent diabetes mellitus: paradoxic patterns

PURPOSE

Non-insulin-dependent diabetes mellitus (NIDDM) in black Americans consists of two variants: one with insulin resistance and one with normal insulin sensitivity. This study examined whether cardiovascular disease risk factors are significantly different between the two variants.

PATIENTS AND METHODS

Twenty-two black patients with NIDDM in near-normoglycemic remission who were receiving no pharmacologic therapy for NIDDM were evaluated for insulin sensitivity by the euglycemic insulin clamp, plasma insulin levels, degree of obesity, glucose metabolism, serum total, high-density lipoprotein (HDL), and low-density lipoprotein (LDL) cholesterol levels, and fasting plasma triglyceride levels.

RESULTS

Fifty-nine percent of these patients had normal insulin sensitivity (glucose disposal rate in response to a 1 mU.kg-1.minute-1 insulin infusion greater than 6.0 mg.kg-1.minute-1). The insulin-sensitive patients were less obese (body mass index [BMI] 26.5 +/- 0.6 versus 30.8 +/- 0.9 kg/m2) and had lower fasting plasma insulin levels (56.9 +/- 7.8 versus 88.0 +/- 6.0 pmol/L), lower serum cholesterol (4.47 +/- 0.30 versus 6.39 +/- 0.26 mmol/L), lower serum LDL cholesterol (2.77 +/- 0.31 versus 4.51 +/- 0.27 mmol/L), and lower fasting plasma triglyceride levels (0.83 +/- 0.08 versus 1.45 +/- 0.16 mmol/L) than the insulin-resistant patients. Serum HDL cholesterol was not different between the two groups and was in the high-normal range (1.31 +/- 0.08 and 1.19 +/- 0.07 mmol/L). Univariate analysis demonstrated that serum total cholesterol, LDL cholesterol, and fasting plasma triglycerides were highly correlated with insulin-mediated glucose disposal and fasting plasma insulin. The differences in insulin sensitivity and lipid profiles were independent of obesity, as they were present in six insulin-resistant and six insulin-sensitive patients matched for BMI.

CONCLUSIONS

Black patients with the insulin-sensitive variant of NIDDM have a low risk factor profile for cardiovascular disease as compared with those with the insulin-resistant variant, who have a high risk factor profile. A high prevalence of the insulin-sensitive variant of NIDDM in the black population might explain the lower prevalence of angina and myocardial infarction in black patients with NIDDM as compared with white patients with NIDDM.